**Course Title:** Mastering Go: From Basics to Advanced Development **Section Title:** Working with the Standard Library: File I/O and Networking **Topic:** Building simple web servers and clients. ### Overview In this topic, we'll explore how to build simple web servers and clients using the Go standard library. Specifically, we'll dive into the `net/http` package, which provides a powerful and flexible way to work with HTTP requests and responses. We'll also touch on the `net` package for basic networking concepts. ### Building a Simple Web Server To build a simple web server in Go, you'll need to import the `net/http` package. Here's a basic example: ```go package main import ( "fmt" "net/http" ) func helloHandler(w http.ResponseWriter, r *http.Request) { fmt.Fprintf(w, "Hello, World!") } func main() { http.HandleFunc("/", helloHandler) fmt.Println("Server is listening on port 8080") http.ListenAndServe(":8080", nil) } ``` This code sets up a basic web server that listens on port 8080 and responds to requests with a "Hello, World!" message. **Key Concepts:** * The `http.HandleFunc` function registers a handler function for a specific URL pattern. In this case, the `helloHandler` function is registered for the root URL (`"/"`). * The `http.ListenAndServe` function starts the server and begins listening for incoming requests. * The `http.ResponseWriter` interface is used to write responses back to the client. ### Handling HTTP Requests In the previous example, we defined a simple handler function that responded to requests with a fixed message. In real-world applications, you'll want to handle different types of requests and respond accordingly. Here's an example that demonstrates how to handle GET and POST requests: ```go package main import ( "fmt" "net/http" ) func handleGet(w http.ResponseWriter, r *http.Request) { if r.Method != "GET" { http.Error(w, "Invalid request method", http.StatusBadRequest) return } fmt.Fprintf(w, "This is a GET request") } func handlePost(w http.ResponseWriter, r *http.Request) { if r.Method != "POST" { http.Error(w, "Invalid request method", http.StatusBadRequest) return } fmt.Fprintf(w, "This is a POST request") } func main() { http.HandleFunc("/get", handleGet) http.HandleFunc("/post", handlePost) fmt.Println("Server is listening on port 8080") http.ListenAndServe(":8080", nil) } ``` This code sets up two separate handler functions for GET and POST requests. **Key Concepts:** * The `http.Request` struct provides information about the incoming request, including the request method. * You can use the `http.Error` function to respond with an error message if the request is invalid. ### Handling Form Data When handling POST requests, you'll often need to extract form data from the request body. Go provides the `encoding/json` and `net/url` packages to help with this. Here's an example that demonstrates how to handle form data: ```go package main import ( "encoding/json" "fmt" "net/http" "net/url" ) type Form struct { Name string Email string } func handleForm(w http.ResponseWriter, r *http.Request) { if r.Method != "POST" { http.Error(w, "Invalid request method", http.StatusBadRequest) return } form := new(Form) err := json.NewDecoder(r.Body).Decode(form) if err != nil { http.Error(w, err.Error(), http.StatusBadRequest) return } fmt.Fprintf(w, "Name: %s, Email: %s", form.Name, form.Email) } func main() { http.HandleFunc("/form", handleForm) fmt.Println("Server is listening on port 8080") http.ListenAndServe(":8080", nil) } ``` This code defines a `Form` struct to represent the form data and uses the `encoding/json` package to decode the request body. **Key Concepts:** * The `encoding/json` package provides functions for encoding and decoding JSON data. * You can use the `net/url` package to parse URL-encoded form data. ### Building a Simple Web Client To build a simple web client in Go, you can use the `net/http` package. Here's an example that demonstrates how to send a GET request: ```go package main import ( "fmt" "io/ioutil" "net/http" ) func main() { resp, err := http.Get("http://example.com") if err != nil { fmt.Println(err) return } defer resp.Body.Close() body, err := ioutil.ReadAll(resp.Body) if err != nil { fmt.Println(err) return } fmt.Println(string(body)) } ``` This code sends a GET request to the example.com URL and prints the response body. **Key Concepts:** * The `http.Get` function sends a GET request to the specified URL. * You can use the `ioutil.ReadAll` function to read the response body. ### Conclusion In this topic, we've explored how to build simple web servers and clients using the Go standard library. We've covered the basics of the `net/http` package, including how to handle HTTP requests, respond to requests, and send form data. We've also touched on the `net` package for basic networking concepts. ### Example Use Cases * Building a simple web API to interact with a database. * Creating a web proxy to forward requests to a different server. * Implementing authentication and authorization for a web application. ### Further Reading * [Go documentation: net/http](https://golang.org/pkg/net/http/) * [Go documentation: net](https://golang.org/pkg/net/) * [Go by Example: Net/HTTP](https://gobyexample.com/net-http) We encourage you to try the examples and experiment with different scenarios. If you have any questions or need help, please leave a comment below.